The mudskipper, Periophthalmodon schlosseri, actively transports NH4+ against a concentration gradient

Periophthalmodon schlosseri can maintain ammonia excretion rates and low le vels of ammonia in its tissues when exposed to 8 and 30 mM NH4Cl, but tissu e ammonia levels rise when the fish is exposed to 100 mM NH4Cl in 50% seawa ter. Because the transepithelial potential is not high enough to maintain t he NH4+ concentration gradient between blood and water, ammonia excretion u nder such a condition would appear to be active. Branchial Na+-K+-ATPase ac tivity is very high and can be activated by physiological levels of NH4+ in stead of K+. Ammonia excretion by the fish against a concentration gradient is inhibited by the addition of ouabain and amiloride to the external medi um. It is concluded that Na+-K+-ATPase and an Na+/H+ exchanger may be invol ved in the active excretion of ammonia across the gills. This unique abilit y of P. schlosseri to actively excrete ammonia is related to the special st ructure of its gills and allows the fish to continue to excrete ammonia whi le air exposed or in its burrow.